Process for the manufacturing of high-density MOS-technology power devices

Abstract: A process for the manufacturing of high-density MOS-technology power devices comprises the steps of: forming a conductive insulated gate layer (8) on a surface of a lightly doped semiconductor material layer (2) of a first conductivity type; forming an insulating material layer (11) over the insulated gate layer (8); selectively removing the insulating material layer (11) and the underlying insulated gate layer (8) to form a plurality of elongated windows (15) having two elongated edges (17) and two short edges (18), delimiting respective uncovered surface stripes (16) of the semiconductor material layer (2); implanting a high dose of a first dopant of the first conductivity type along two directions which lie in a plane transversal to said elongated windows (15) and orthogonal to the semiconductor material layer (2) surface, and which are substantially simmetrically tilted of a first prescribed angle (A1,A2) with respect to a direction (T) orthogonal to the semiconductor material layer (2) surface, the first angle (A1,A2) depending on the overall thickness of the insulated gate layer (8) and of the insulating material layer (11) to prevent the first dopant from being implanted in a central stripe of said uncovered surface stripes (16), to form pairs of heavily doped elongated source regions (6) of the first conductivity type which extend along said two elongated edges (17) of each elongated window (15) and which are separated by said central stripe; implanting a low dose of a second dopant of a second conductivity type along two directions which lie in said plane, and which are substantially simmetrically tilted of a second prescribed angle (A3,A4) with respect to said orthogonal direction (T), to form doped regions of the second conductivity type each comprising two lightly doped elongated channel regions (5) extending under the two elongated edges (17) of each elongated window (15); implanting a high dose of a third dopant of the second conductivity type substantially along said orthogonal direction (T), the insulating material layer (11) acting as a mask, to form heavily doped regions (4) substantially aligned with the edges (17,18) of the elongated windows (15).

Description: The present invention relates to a process for the manufacturing of high-density MOS-technology power devices, such as power MOSFETs and Insulated Gate Bipolar Transistors (IGBTs), and to a high-density MOS-technology power device integrated structure thus obtained. It is known that MOS-technology power devices are composed of a plurality of elementary functional units integrated in a semiconductor chip; each elementary functional unit forms an elementary vertical MOSFET which contributes for a respective fraction to the overall power device current. Conventionally, the elementary functional units are represented by polygonal-plan elementary cells, comprising a polygonal-plan P type region (called "body region"), formed in an N - semiconductor layer and comprising in turn a central P + region (called "deep body region") and a lateral P - channel region; an N + polygonal annular source region is formed inside the P type body region. The polygonal annular portion of the body region comprised between the external edge of the N + source region and the edge of the body region is covered by a conductive layer (polysilicon gate) superimposed on an insulating layer (gate oxide) and forms a channel region of the elementary MOSFET. The polysilicon layer is covered by a dielectric layer, in which a contact window is opened to allow a superimposed conductive layer (source electrode) to come into contact with the N + source region and the portion of the body region delimited by the inner edge of the N + source region; this is necessary to prevent a parasitic bipolar junction transistor (with emitter, base and collector represented by the source region, the deep body region and the N - semiconductor layer) from being triggered on. A conventional manufacturing process substantially provides for: epitaxially growing the N - semiconductor layer over a heavily doped substrate of the N conductivity type, in the case of a power MOSFET, or of the P conductivity type, in the case of an IGBT; maskedly implanting and diffusing a heavy dose of a P type dopant into selected regions of the N type layer, to form the deep body regions of the elementary cells (first mask); thermally growing a thin oxide layer over the surface of the N type layer; depositing a polysilicon layer over the thin oxide layer; selectively etching the polysilicon layer and the thin oxide layer around the deep body regions (second mask); implanting a low dose of a P type dopant using the polysilicon and oxide layers as a mask; diffusing the P type dopant to form channel regions extending under the thin oxide layer; maskedly implanting a heavy dose of an N type dopant into the deep body and channel regions of the cells to form annular source regions (third mask); depositing a dielectric layer over the polysilicon layer; selectively etching the dielectric layer to open in it contact windows to the deep body regions and to the source regions (fourth mask); depositing a metal layer over the dielectric layer. The necessity of respecting the mask alignement rules in two directions (parallel to the sides of the elementary cells) severely limits the possibility of scaling down the dimensions of the cells, to increase the scale of integration. In the co-pending European Patent Application No. 94830288.0 a MOS-technology power device is disclosed wherein the elementary functional units are represented by stripes much more elongated in a longitudinal direction (let's call it "Y") on the surface of the N- semiconductor layer than in a transversal direction ("X"); the source region comprises two regions elongated in the Y direction, and a plurality of transversal portions. In such a structure it is no longer necessary to respect the alignement rules of the contact mask with respect to the source mask along the X direction, since the transversal portions of the source region guarantee the short circuit between the source region and the deep body region even if the contact window opened in the dielectric layer has, in the X direction, the minimum dimension ("f") allowed by the optical resolution of the photolitographic apparatus employed. In the above mentioned patent application, it is said that the minimum dimension Lsmin of the stripe in the X direction is given by: Lsmin = f + 2a where a is the minimum distance between the edge of the polysilicon layer and the edge of the contact window which, taking into account the alignement tolerance between the polysilicon mask and the contact mask, guarantees that the polysilicon gate is insulated from the source electrode. Actually, another limit exists to the reduction of the dimension of the stripes along the X direction. The source mask is partially provided by the polysilicon and gate oxide layers, but further comprises a plurality of isles of photoresist over the middle portion of the stripes; this prevents the dimension of the stripes in the X direction from being below a value Lsmin' given by: Lsmin' = Apmin + 2s where Apmin is the minimum X dimension of the photoresist isles (minimum aperture in the dielectric layer) and coincide with the minimum dimension f allowed by the optical resolution of the photolitographic apparatus, and s is the minimum X dimension of each elongated portion of the source region, taking into account the alignment tolerance between the polysilicon mask and the source mask (i.e. between the edges of the polysilicon layer and the edges of the photoresist isles). By way of example, if for the source region a minimum dimension of 0.3 um in the X direction must be guaranteed, and the mask alignment tolerance is + 0.2 um, s must be at least 0.5 um, and this must be considered the minimum dimension during the source mask layout design. If Lsmin < Lsmin', the actual limit to the reduction in the X dimension of the stripes is given by Lsmin'. JP-A-4065132 discloses a process for manufacturing double diffusion type MOSFET devices. A body region is firstly implanted in a central portion of a semiconductor layer and channel regions and source regions are then formed by tilted implantation. US-A-4417385 relates to processes for manufacturing insulated gate power semiconductor devices, particularly MOSFET devices. Tilted dopant implantation steps are used to form base and source regions. Thermal diffusion of dopants is used to appropriately locate the base and source regions. Shorting extension of the base regions are then formed between the source regions. EP-A-0481153 discloses a process for manufacturing power MOS transistors in which heavily doped body regions are located under source regions and above lightly doped channel regions in contact with an underlying semiconductor layer. In view of the state of the art disclosed, it is an object of the present invention to provide a process for the manufacturing of MOS-technology power devices which allows to obtain higher scales of integration with respect to the above mentioned prior art. According to the present invention, such object is attained by means of a process for the manufacturing high-density MOS-technology power devices, as set forth in claim 1. In the process according to the present invention, the implantation mask for the source regions coincides with the implantation mask for the channel regions and for the deep body regions of the elementary functional units; this is made possible by the use of implanting apparatus that allow the dopant to be implanted along directions tilted of angles ranging from 0° to 60° with respect to the direction orthogonal to the surface of the material wherein the dopant is to be implanted. Consequently, the tolerance on the dimension of the source regions is not determined by the mask alignment rules, but substantially by the tolerance on the thickness of three Layers (gate oxide, polysilicon gate, insulating oxide), which is at least one order of magnitude lower. The dimension of the source regions is more finely controlled, and narrower elementary functional units can be formed. Also according to the present invention, a high-density MOS-technology power device as set forth in claim 18 is provided. The features and advantages of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is a top plan view of a portion of a MOS-technology power device according to the present invention; Figure 2 is a cross-sectional view of the power device taken along the line II-II of Figure 1; Figures 3 to 10 are cross-sectional views taken along the line II-II of Figure 1, showing intermediate steps of a manufacturing process according to the invention; Figures 4A, 6A and 8A show top plan views of the power device just after the steps shown in Figures 4, 6 and 8, respectively. As shown in Figures 1 and 2, respectively in top-plan and cross-sectional views, a high-density MOS-technology power device according to the present invention (a power MOSFET in this example) comprises in a per-se known manner an N - semiconductor layer 2 superimposed on an N + semiconductor substrate 1. The N - Layer 2 forms a common drain region for the power MOSFET, and the N + substrate 1 provides a low resistance contact to the N - layer 2. A metal layer 14 covers the bottom surface of the N + substrate 1 and forms a drain electrode D of the power MOSFET. P type elongated regions 3, substantially of rectangular shape, are formed in the N - layer 2; each of said P type regions 3 comprises a P + deep body region 4 laterally surrounded by two P - body regions 5. All the P type regions 3 extend along a longitudinal direction (Y, in Figure 1) on the top surface of the N - layer 2. The dimension of the P type regions 3 in the Y direction is much greater (at least two orders of magnitude) than their dimension in a transversal direction (X, in Figure 1). Two N + source regions 6 are provided inside each P type region 3, substantially in the P + deep body region 4; the N + source regions 6 are also elongated in the Y direction, but do not extend for the whole length of the P type region 3, for the reasons which will be explained later on. Selected portions of the surface of the N - layer 2 are covered by a conductive insulated gate layer 8 comprising an insulating layer 9 (typically a thin silicon dioxide film also called "gate oxide"), and a conductive layer 10 (typically a polysilicon layer); the polysilicon Layer 10 forms a gate electrode of the power MOSFET. The insulated gate layer 8 is covered by a further insulating layer 11, such as silicon dioxide; the insulated gate layer 8 and the insulating layer 11 extend over the P - body regions 5 to the edges of the N + source regions 6. The edges of the insulated gate layer 8 and of the insulating Layer 11 are substantially aligned with the external edges of the N + source regions 6, and with the edge of the P + deep body regions 4. The edges of the insulated gate layer 8 and of the insulating layer 11 are sealed by sidewall spacers 12 of insulating material (e.g. silicon dioxide), which partially overlap the N + source regions 6. A metal conductive Layer 13 (typically aluminum) covers the insulating layer 11 and contacts all the N + source regions 6 and the P + deep body regions 4; such a conductive layer forms a source electrode S of the power MOSFET. Each P type region 3 represents an elementary functional unit (an elementary vertical MOSFET in this example), which is connected in parallel with all the other units and contributes for a given fraction to the overall power MOSFET current. A manufacturing process according to the present invention starts from a N + semiconductor substrate 1 having a dopant concentration of about 10 19 atoms/cm 3 . An N - semiconductor layer 2 is epitaxially grown over the N + substrate 1; the N - layer 2 has resistivity ranging from 0.5 Ohm x cm to 50 Ohm x cm, and thickness ranging from 3 µm to 50 µm, depending on the breakdown voltage desired for the power MOSFET. A thick oxide layer (not shown in the drawings) is grown over the N - layer 2, to form the edge structure of the power device chip; such thick oxide is removed from the so-called "active area" of the power device, i.e. the portion of the chip wherein the elementary MOSFETs are to be integrated. An insulated gate layer 8 is then formed over the surface of the N - layer 2 in correspondance of the active area of the device; the insulated gate layer 8 comprises a thermally-grown thin oxide layer 9 and a polysilicon layer 10. A further thermal oxide layer 11 is formed over the polysilicon layer 10 (Fig. 3). The oxide layer 11 and the insulated gate Layer 8 are then selectively removed to open windows 15 having substantially a rectangular shape with two elongated edges 17 along the Y direction, and two short edges 18 along the X direction; consequently, uncovered surface stripes 16 of the N - layer 2 are obtained (Figs. 4 and 4A). This step is performed in a per-se knwon way by covering the oxide layer 11 with a photoresist (not shown), and removing it from the regions of the oxide layer 11 which are to be exposed to the action of an etchant whith high selectivity towards silicon dioxide and polysilicon with respect to the photoresist material. The windows 15, and thus the uncovered surface stripes 16, are much more elongated in the Y direction than in the X direction (at least two orders of magnitude). A high dose of an N type dopant is then selectively implanted into the N - Layer 2, along a direction which lies in an orthogonal plane transversal to the stripes 15 (such as the plane of Figures 2-10), and which is tilted of an angle A1 with respect to a direction T orthogonal to the surface of the N - layer 2; the tilt angle A1 can have a value ranging from 45° to 60°, depending on the overall thickness of the insulated gate layer 8 and the oxide layer 11, so that the oxide layer 11 and the insulated gate layer 8 act as an implantation mask to allow the dopant being implanted only in regions near one of the edges 17 of the windows 15, shielding the middle of the stripes 16. If for example the overall thickness of the layers 9, 10 and 11 is 0.5 µm, a tilt angle A1 of 60° allows to form, inside each uncovered surface stripe 16, an N + stripe 6 of about 0.2-0.25 µm in the X direction and extending in the Y direction along said edge 17 of the windows 15 (Fig. 5). The previous step is repeated tilting the dopant ion beam of an angle A2, substantially simmetrical to A1, to form in a same manner another N + stripe 6 along the opposite edge 17 of the windows 15; thanks to the shielding action of the layers 11 and 8, a central portion of the surface stripes 16 does not receive the N type dopant (Figs. 6 and 6A). Since implanting apparatus are commercially available which allow a fine control of the angle of tilting of the dopant ion beam in the range 0°-60°, the tolerance on the dimension along the X direction of the N + source regions 6 substantially depends on the tolerance of the overall thickness of the oxide and insulated gate layers 11 and 8; it is known that the tolerance on the thickness of the grown or deposited layers is much narrower than the alignment tolerance between photolitographic masks: while in fact in current photolitographic apparatus a tolerance of about 0.2 µm must be taken into account when two mask are to be aligned, thickness tolerances of 2-3% for layers of 500 Angstrom thickness are easily attainable. This technique allows to reduce the dimension along the X direction of the source regions 6 (from the current 1-3 µm to 0.2 - 0.25 µm), and of the non-implanted central portion of the surface stripes 16. A photoresist mask 19 (shown in Fig. 6A in dash-and-dot lines) prevents the dopant from being implanted into the regions of the uncovered surface stripes 16 near the short edges 18 of the windows 15; the N + source regions 6 do not extend therefore for the whole length of the edges 17 of the windows 15. A P type dopant, such as boron, is then selectively implanted into the N - layer 2 along a direction which lies in said transversal plane, and which is tilted of an angle A3 with respect to the direction T; differently from the previous two steps, in this case the tilt angle is chosen, together with the implantation energy, to make the dopant penetrate under one edge 17 of the windows 15; a suitable angle can be in the range 35°-60°, preferably 45°. In each uncovered surface stripe 16 a P - type stripe 5, extending in the Y direction, is thus formed around the N + stripe 6, and extend under the insulated gate layer 8 to form a channel region of the elementary MOSFETs (Fig. 7). The implantation dose is generally Low, and depends on the desired threshold voltage of the power MOSFET; the implantation energy must be higher than that used for the implantation of N + source regions 6. The previous step is repeated tilting the dopant ion beam of an angle A4, substantially simmetrical to A3, to form in a same manner another P - type stripe 5 along the opposite edge of each window 15 (Figs. 8 and 8A). Differently from the N + source regions 6, the P - regions 5 extend for the whole length of the uncovered surface stripes 16; the absence of the N + source regions 6 near the short edges 18 of the windows 15 prevents a channel short-circuit from occurring (since in fact no channel regions are provided under the short edges of the windows 15, if the source regions 6 extend for the whole length of the uncovered surface stripes 16 they would be in direct contact with the N - layer 2, thus causing a source-drain short-circuit). A high dose of a P type dopant is then implanted along the orthogonal direction T at a relatively high energy (100-200 KeV or greater), the oxide and insulated gate layers 11 and 8 acting as a mask so that after the implantation P + deep body regions 4 are formed which are substantially self-aligned with the edges 17 and 18 of the windows 15; the P + deep body regions 4 extend therefore under all the N + source regions 6 which, according to the way in which they are formed, are also self-aligned with the elongated edges 17 of the windows 15 (Fig. 9). The implantation energy is chosen in such way as to locate the peak dopant concentration just under the source regions 6, and the dopant dose is chosen in such a way as the dopant concentration at the surface is of the order of 10 18 atoms/cm 3 . A high temperature, low time duration thermal step is at this point performed to activate the dopants and to partially eliminate the defects caused by the implanted dopants. Such a step is performed in Rapid Thermal Annealers (RTAs), and does not cause significant dopant redistribution; the doping profiles and dimensions of the various semiconductor regions are therefore not affected. Oxide sidewall spacers 12 are then formed along the edges 17 and 18 of the windows 15 by means of a CVD deposition at low temperature (300-400°C) (Fig. 10). A metal layer 13 is then deposited over the oxide layer 11, to form a source electrode S of the power MOSFET; the oxide layer 11 and the sidewall spacers 12 electrically isolate the polysilicon gate layer 10 from the metal layer 13. A further metal layer 14 is formed on the bottom surface of the substrate 1 to form a drain electrode D. It appears that the described process involves only one mask (or two masks, considering the mask for the photoresist layer 19) with respect to the four masks required by the conventional processes. The P + deep body regions 4, the P - regions 5 and the N + source regions 6 are all self-aligned with the litographic mask used to define the windows 15 in the oxide layer 11 and in the insulated gate layer 8. A distinctive feature of the process according to the invention with respect to the conventional processes is that it does not involve thermal steps with high thermal budgets; this has several advantages, which will be now discussed. Firstly, the thickness of the gate oxide layer 9 can be reduced from the current 350-500 Angstroms to 100-200 Angstroms. Secondly, the depth of the various semiconductor regions is determined by the implantation energy instead of the dopant diffusion process; in particular, the P + deep body region 4 can be as shallow as 0.6 µm (instead of the current 3-4 µm); since the breakdown voltage of the power MOSFET depends on the thickness of the portion of N- epitaxial layer 2 under the P+ deep body regions, the thickness of the epitaxial layer can be reduced of 2.4-3.4 µm. Thirdly, the channel regions 5 are shorter than in conventionally-manufactured power device structures; in this way, not only the output resistance (Ron) of the power device is reduced, but further an increase in the integration scale of elementary functional units on a same chip is allowed. Since the process according to the present invention does not involve thermal processes with high thermal budgets, the previously described sequence of steps could be altered without affecting the resulting structure; more precisely, the implantation steps leading to the formation of the source regions, of the channel regions and of the deep body regions can be interchanged. It would therefore be possible, for example, to initially form the deep body regions 4, then the channel regions 5, and finally the source regions 6. The process according to the present invention applies as well to the fabrication of IGBTs, the only difference being the conductivity type of the substrate 1.

Claims: Process for the manufacturing of high-density MOS-technology power devices, in the absence of thermal steps which would affect doping profiles or dimensions of the various device regions comprising the steps of: a) forming a conductive insulated gate layer (8) on a surface of a lightly doped semiconductor material layer (2) of a first conductivity type; b) forming an insulating material layer (11) over the insulated gate layer (8); c) selectively removing the insulating material layer (11) and the underlying insulated gate layer (8) to form a plurality of elongated vertical-wall windows (15), having substantially a rectangular shape and having two elongated edges (17) and two short edges (18), delimiting respective uncovered surface stripes (16) of the semiconductor material layer (2); d) implanting a high dose of a first dopant of the first conductivity type along two directions which lie in a plane transversal to said elongated windows (15) and orthogonal to the semiconductor material layer (2) surface, and which are substantially symmetrically tilted of a first prescribed angle (A1,A2) with respect to a direction (T) orthogonal to the semiconductor material layer (2) surface, the first angle (A1,A2) depending on the overall thickness of the insulated gate layer (8) and of the insulating material layer (11) to prevent the first dopant from being implanted in a central stripe of said uncovered surface stripes (16), to form pairs of heavily doped elongated source regions (6) of the first conductivity type which extend along said two elongated edges (17) of each elongated window (15) and which are separated by said central stripe wherein the implantation of said first dopant is prevented in regions near to the short edges (18) of said elongated windows (15); e) implanting a low dose of a second dopant of a second conductivity type along two directions which lie in said plane, and which are substantially symmetrically tilted of a second prescribed angle (A3,A4) with respect to said orthogonal direction (T), to form doped regions of the second conductivity type each comprising two lightly doped elongated channel regions (5) extending under the two elongated edges (17) of each elongated window (15) the implantation energy being higher then that valid in step (d); f) implanting a high dose of a third dopant of the second conductivity type substantially along said orthogonal direction (T), the insulating material layer (11) and the underlying insulating gate layer (8) acting as a mask, to form heavily doped body regions (4) substantially aligned with the edges (17,18) of the elongated windows (15) and extending under said source regions (6), said third dopant being implanted at an energy sufficiently high so that the peak concentration of the third dopant is located under the source regions (6); g) forming insulating material sidewall spacers (12) at the edges (17,18) of said elongated windows (15); and h) forming a conductive material layer (13) over the insulating material layer (11). Process according to claim 1, characterized in that said first prescribed angle (A1,A2) is chosen in the range 45°-60°. Process according to anyone of claims 1 to 2, characterized in that said second prescribed angle A3,A4) is chosen in the range 35°-60°. Process according to anyone of claims 1 to 3, characterized in that said third dopant is implanted at an energy equal to or greater than 100 KeV. Process according to claim 1, characterized in that the step e) is performed before the step d). Process according to claim 5, characterized in that the step f) is performed after the step e) and before the step d). Process according to claim 1, characterized in that the step f) is performed before the step d). Process according to claim 7, characterized in that the step e) is performed after the step f) and before the step d). Process according to claim 1, characterized in that the step f) is performed before the step e). Process according to claim 1, characterized in that said insulated gate layer (8) is formed by growing a thin oxide layer (9) over the surface of the semiconductor material layer (2), and covering the thin oxide layer (9) with a further conductive material layer (10). Process according to claim 1, characterized in that said semiconductor material layer (2) is an epitaxial layer grown over a heavily doped semiconductor material substrate (1). Process according to claim 11, characterized in that said semiconductor material substrate (1) is doped of the first conductivity type, the power device being a power MOSFET. Process according to claim 11, characterized in that said semiconductor material substrate (1) is doped of the second conductivity type, the power device being an IGBT. Process according to anyone of the preceding claims, characterized in that said first conductivity type is the N type, and said second conductivity type is the P type. Process according to anyone of claims 1 to 13, characterized in that said first conductivity type is the P type, and said second conductivity type is the N type.

Inventors: FERLA GIUSEPPE, FRISINA FERRUCCIO, FERLA, GIUSEPPE, FRISINA, FERRUCCIO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, H01L29